Abstract: Objective To analyze the association between body mass index (BMI) and risk of small for gestational age(SGA).Methods Subjects were 5 084 pregnant women who had joined in the Ma'anshan Birth Cohort Study and delivered between October 2008 and October 2010.All the pregnant women completed the maternal and infant health record forms in their first prenatal care and we collected information on socio-demographic characteristics,pre-pregnancy BMI,and singleton live births from the forms.Chi-square analysis and multivariate stepwise regression analysis were performed to analyze the effect of pre-pregnancy BMI on SGA.Results Chi-square tests showed that there were statistically significant differences in SGA incidence among the women with diffenent residence place,status of gestational hypertension,and pre-pregnancy BMI(P<0.05 for all).The overall rate of SGA was 3.0%.The stepwise multinomial regression analyses showed that pre-pregnancy BMI less than 18.5 was a risk factors for SGA (risk ratioRR=1.63,95%confidence interval95%CI=1.12-2.37).Conclusion Low pre-pregnancy BMI could induce SGA and pregnant women should pay attention to their BMI and to reduce the risk of SGA.